Hillary Clinton: Seen as a potential candidate since first elected senator for New York in 2000. A fundraising powerhouse, but ambiguous about the Iraq war, which could hurt her. May make a declaration this week.

Barack Obama: Junior senator from Illinois and a rising star since his electrifying speech to the 2004 party convention. His lack of experience is made up for by his appeal to audiences from New Hampshire to Kenya, where his father hails from. Opposed Iraq war.

John Edwards: Veteran of the campaign trail as John Kerry's 2004 running mate. He has staked out ground on the left, with an anti-poverty platform. Opposed Iraq war.

Republicans

John McCain: Former navy pilot and senator from Arizona owes his military credentials to the years spent in the "Hanoi Hilton" after being shot down in Vietnam. Backs George Bush on the troop surge.

Mitt Romney: A Mormon and son of a former Michigan governor, Romney rose to prominence for organising the 2002 Salt Lake City Olympics. As a Republican governor of liberal Massachusetts, he approved a ban on assault weapons and made no move to restrict abortion rights.

Sam Brownback: Kansas senator expected to make a formal announcement on Saturday, and will run on the issues that move the Republican base. Opposes abortion and stem cell research, and what he calls the "homosexual agenda", and supports creationism in state schools

Rudy Giuliani: Former New York city mayor's interest became clear this month after a strategy document was leaked. Moderate image may hurt him among primary voters.